it is known that some types of breakdowns on an aircraft, such as undesired movements of control surfaces, incorrect lateral centring, undesired extension of thrust reversers or outside conditions such that a frost accretion difference between the right wing and the left wing, can generate a lateral dissymmetry of the aircraft. By lateral dissymmetry, it is meant the default of symmetry between right side and left side of the aircraft, with respect to the median vertical plan of said aircraft.
Such a lateral dissymmetry results in the following effects on the behaviour of an aircraft, in particular a cargo aircraft:                it generates a roll movement which is generally compensated for by control surfaces of the roll axis (for example ailerons and spoilers); and        it generates a yaw movement which is generally compensated for by control surfaces of the yaw axis (for example the rudder unit).        
The direction and magnitude of such movements depend on the side of dissymmetry and the type of breakdowns or the outside environmental conditions at the origin of such movements. This dissymmetry is compensated for, during the flight, either automatically by an auto-flight device of the aircraft, either manually by the pilot using the control members. Such compensation generates an increase of the drag in addition to the additional drag already induced by the dissymmetry. Such increase is proportional to the nature and level of the dissymmetry to be corrected and which results in an increase of fuel consumption.
This phenomenon, if it remains for a significant time, results in an overconsumption of fuel and, in extreme cases, a temporary loss of control of the aircraft and a diversion thereof.
The usual solutions in order to be protected against breakdowns such as mentioned above are based on monitoring actuators of control means.
Yet, these usual solutions are specific to different types of breakdowns, for example engine breakdowns or uncontrolled deflections of the control surfaces such as ailerons. Such usual solutions are not capable of detecting all breakdowns or changes of outside environmental conditions which can lead to an unusual behaviour of the aircraft, for example a lateral dissymmetry.
In particular, from document FR-2,897,592, a method and device are known for detecting a lateral dissymmetry which appears when the roll control surfaces of an aircraft are positioned near the abutments thereof, without control of the pilot of the aircraft. This usual detection method includes the following steps of:    a) determining the current value of a control parameter which is representative of all the roll control surfaces of the aircraft;    b) comparing such current value with a predetermined reference value;    c) determining a current deflection angle of a lateral stick of the aircraft, which is likely to be actuated by a pilot of the aircraft for controlling the roll control surfaces;    d) comparing such current deflection angle with a predetermined angle value; and,    e) if at a time the current value of the control parameter is higher than the reference value and the current deflection angle is higher than the angle value, transmitting a visual alarm signal on a visualisation display screen of the aircraft cockpit, which indicates the detection of a lateral dissymmetry of the aircraft.